The present invention relates to fluid coupling devices of the type which transmit torque from an input coupling member to an output coupling member by means of a viscous fluid.
Although the present invention may be used advantageously in fluid coupling devices having many different configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Conventionally, fluid cooling devices include an output coupling member defining a fluid chamber, a partition separating the fluid chamber into a fluid reservoir chamber and a fluid operating chamber, and an input coupling member rotatably disposed within the operating chamber. When the operating chamber is substantially filled with viscous fluid, rotation of the input coupling member transmits torque to the output coupling member by means of viscous shear (engaged mode). When operation in the engaged mode is not required (e.g., when engine cooling is not required), a valve member covers the fill opening, and fluid is pumped out of the operating chamber back into the reservoir chamber such that the coupling device operates in the disengaged mode.
For many years, those skilled in the art have recognized the desirability of being able to manually engage a viscous fan drive in the event the vehicle overheats. Also, if there is a malfunction in the cooling system, it is desirable to be able to manually engage the fan drive for diagnostic purposes, to try to determine the actual source of the malfunction, rather than replacing a component such as the viscous fan drive, and then determining subsequently that the fan drive was not defective in any way.
Despite the recognized need for manual engagement capability for viscous fan drives, the prior art has, prior to the present invention, provided only devices of the type shown in U.S. Pat. No. 4,320,723. The device in the cited patent achieves manual engagement by mechanically locking the housing of the fan drive to the input shaft. As will be understood by those skilled in the art, the result of the arrangement in the cited patent is that the fan will be driven at the speed of the input shaft, whereas during normal operation (torque transmission by viscous shear), the fan speed is limited to some predetermined, maximum speed by the inherent, limited torque-transmitting capability of the viscous fluid. It is important to note that viscous fan drives and radiator cooling fans are designed and sized based upon the assumption that the fan and fan drive will never operate above the predetermined, maximum speed which can be achieved as a result of the viscous shear. If such a fan drive has its output mechanically locked to the input shaft, the result is substantially greater torque loading on the fan, which can damage the fan blades as well as the spider, and substantially greater torque loading on the fan drive which can damage the bearings, the shaft-clutch connection, etc. The concern over such damage is even greater in the case of crank-mounted fan drives wherein a substantial amount of torsional vibration is transmitted to the input shaft of the fan drive. When such a crank-mounted fan drive is transmitting torque by means of viscous shear, the viscous fluid effectively damps much of the torsional vibration. However, if such a fan drive has its output mechanically locked to the input shaft, as has been taught by the prior art, the torsional vibration is transmitted to the housing of the fan drive and to the radiator cooling fan.